The pneumatic fender is installed in a quay in a port or on the side of a ship by being anchored with a guy rope (for example, refer to Japanese Unexamined Patent Application Publication No. 2007-168749A). For example, when performing an operation of passing crude oil or the like between two ships at sea, a pneumatic fender is installed between these ships. The tensile force generated in the guy rope varies during such an operation or when installing or removing the pneumatic fender. A guy rope able to sufficiently withstand the tensile force which it is assumed will be generated is selected as the guy rope to be used; however, breakages may occur due to the generation of excessive tensile forces at least equivalent to the assumed tensile force. When the guy rope breaks, extensive damage is caused, forcing interruptions to the operation or the like.
However, in the related art, there was no means for determining the tensile force generated in the guy rope with high accuracy. Therefore, it was necessary to take countermeasures to prevent breakage of the guy rope such as using an excessively thick guy rope set to a high safety factor.